Environmentally harmful species in the exhaust gas emitted from an internal combustion engine, such as hydrocarbons (HC), carbon monoxide (CO), particulate matters (PM), and nitric oxides (NOx) are regulated species that need to be removed from the exhaust gas. In lean combustion engines, due to the existence of large amount oxygen excess, passive means without extra dosing agents, such as that using a three-way catalyst, normally are not able to effectively remove the oxidative specie NOx, as that in most of spark-ignition engines. To reduce NOx in lean combustion engines, a variety of active means with reducing agents (reductants) being dosed in exhaust gas are developed. In these technologies, normally the reductant is metered and injected into the exhaust gas, and the result mixture flows into a SCR (Selective Catalytic Reduction) catalyst, where the reducant selectively reacts with NOx generating non-poisonous species, such as nitrogen, carbon dioxide, and water.
A variety of reductants, such as ammonia (NH3), HC, and hydrogen (H2) can be used in SCR systems. Among them, ammonia SCR is used most broadly due to high conversion efficiency and wide temperature window. Ammonia can be dosed directly. However, due to safety concerns and difficulties in handling pure ammonia, normally urea solution is used in ammonia SCR systems. Urea can be thermally decomposed and hydrolyzed to ammonia in exhaust gas.
Typically, in a SCR control system, the required ammonia dosing rate is calculated in an ECU (Engine Control Unit) 150. Then according to the urea-to-ammonia ratio, the required urea flow rate is calculated and the dosing rate command is sent to a dosing system, where urea solution is metered and injected into exhaust gas. Generally, similar to fueling control, there are two methods in metering reductant. One method is using a metering pump, with which the reductant flow rate is precisely controlled by controlling the pumping rate. The other method is more like that used in a common rail fueling control system. In this method, a pressure is built up and maintained constant in a reductant rail or buffer, and reductant flow rate is controlled by adjusting the opening time of an injector, which is fluidly connected to the buffer, in a repeating control cycle.
Atomization of reductant is important to SCR conversion efficiency, especially in a urea SCR system, where dosed urea needs to be thermally decomposed and hydrolyzed to ammonia and the heat energy provided by exhaust gas is limited. In the first reductant metering method, though the control is simple, the reductant pressure is not controlled. Therefore, to have a good atomization, in addition to having a well-designed nozzle facilitating atomization, normally the reductant dosing needs to be mixed with an extra air supply providing a continuous air flow. The requirements of a continuous air flow and a precisely controlled metering pump limit the application of this method. The second reductant metering method doesn't need an extra air supply to facilitate atomization, since under high pressure, injected reductant from a well-designed nozzle has good atomization. However, in this method, due to the requirement of pressure control, typically a liquid pump, such as a membrane pump, driven by a motor, is needed in establishing and maintaining the rail pressure, and a complex motor control system is required.
Additionally, to avoid frozen reductant under low ambient temperature, reductant residue inside the dosing system need to be purged before the dosing system is shut off. In a system using the first reductant metering method, air supply can be used to push the reductant residue back to tank, while in that using the second method, an extra reductant flow control is needed to drive the reductant residue back. In dosing systems which have reductant residue in connection lines, line heating means are also required. Different from reductant tank heating control, line heating is a distributed heating and it is hard and costly to use closed-loop controls. Except using special PTC (Positive Temperature Coefficient) heaters, heating power and line durability need to be carefully balanced to avoid damage caused by locally over-heating.
For decreasing the complexity of a reductant dosing system while at the same time achieving good performance, a primary object of the present invention is to provide a reductant dosing apparatus using air driven hydraulic pumps with a simple pressure control to build up and maintain a high pressure in a rail. The air driven hydraulic pump doesn't have a motor inside and, therefore, doesn't need electrical energy and a complex motor control to drive it. Neither the air driven hydraulic pump needs a continuous air supply.
A further object of the present invention is to provide a method controlling dosing rate insensitive to variations in reductant pressure, so that accurate dosing rate is obtained under varying reductant pressure.
Another object of the present invention is to provide a dosing apparatus with an air driven hydraulic pump using compressed air generated from an engine turbo, so that no extra air source is required.
Yet another object of the present invention is to provide a control means using compressed air to drain reductant residue back to tank when a dosing process completes.
Yet another object of the present invention is to provide a dosing apparatus with an air driven hydraulic pump positioned inside a reductant tank, thereby no extra heating means other than tank heating is needed for the pump.